We have completed an examination of the effects of adult infraorbital (IO) transection on the physiology of trigeminal ganglion cells. Four hundred fifty-eight ganglion cells were recorded with glass or tungsten microelectrodes at least at 60 days after IO nerve transection. We have found that this injury results in 1) an increase in the number of nociceptive units, 2) a shift in the relative percentages of cells sensitive to innocuous stimuli, 3) a number of cells with abnormal receptive fields and 4) a significant alteration of maxillary intradivisional topography. The percentage of total IO vibrissa units decreased from a normal value 77% to 28%, while the guard hair units increased from 12% in the normal animals to 36%. At the same time, the percentage of IO nociceptive units increased from the normal value of 6% to 20%. We also encountered 15 units which were driven by the IO stimulating electrode but had not peripheral receptive field (none of our normal units exhibited this characteristic). In addition to an increase in the number of nociceptive and guard hair units, adult IO nerve transection also resulted in a number of trigeminal ganglion cells with abnormal receptive fields. These unusual units included five cells with discontinuous receptive fields (two responding to deflection of more than on vibrissa), a unit sensitive to movement of a vibrissa and surrounding guard hairs and eight cells that responded only to deep pressure in the region of the IO foramen.